Why should you crossover a subwoofer below 100Hz?
The crossover frequency of a subwoofer filter has to be decided very wisely. It is a common practice to crossover small drivers at 150Hz. But making the subwoofer filter at 150 Hz makes the system a bit boomy. This is because the audio spectrum has more signal power in the higher frequencies say above 100Hz and comparitively low power for sub frequencies below 100Hz. So making a cutoff @ 150Hz or so will never work good for subwoofers. Looking at the frequency response we can find a huge peak in the frequencies around 120Hz and the response will not look flat. This huge peak is the cause for all boominess. But what if a subwoofer filter has a cut off at 100Hz? At this cutoff point the power of midbass frequencies get cutoff very rapidly by the fourth order filter. You still have output from the subwoofer producing 100-150Hz. But the peaks are well controlled. Thus you get a nearly flat response of the subwoofer till 150Hz. Some top brands adapt to this technique
